I'm maybe going to build a battery powered embedded unit. It would be necessary to determine the battery's condition at power on to alert the user that he must change the battery.
The unit must know that it can activate a relay at least once during power on. That is the purpose of the battery test.
The unit is going to have a microcontroller.
Any ideas of how to make that test? Maby measure the battery voltage when the unit applies a resistive load to it. And then determine the V and the Ri of the battery.
Should I use two batteries so I can measure one at a time at some interval?
It's for a safety critical unit.